Project Description Systemic sclerosis/scleroderma (SSc) is a connective tissue disease characterized by vascular, immune and fibrotic changes in the skin and internal organs. SSc patients are known to have high prevalence of osteopenia, but the pathophysiological mechanism causing this osteoporosis in SSc patients is unknown. Systemic infusion of mesenchymal stem cells (MSCs) yields a therapeutic effect in a variety of autoimmune diseases; however, the reasons behind this MSC-based therapy are not fully understood. As a secretome, MSCs release large amounts of cytokines and small vesicles for immunoregulation and communication with surrounding cells. The goal of this proposal is to investigate how small vesicles are controlled by a set of membrane traffic proteins for cell component reuse in MSC transplantation (MSCT). My preliminary data show that MSCT significantly rescues disease phenotypes in SSc mice through rebuilding tissue homeostasis and lineage commitment of host MSCs. miRNA-151-5p is reused by recipient MSCs through exosome trafficking resulting in markedly recovered lineage selection of host MSCs. A micro-vesicle trafficking complex assembled in donor MSCs regulated exocytosis for exosome-mediated miRNA transfer. In addition, developmental endothelial locus-1 (Del-1), an immune suppressive molecule, expresses in MSCs and releases via exosomes to inhibit neutrophil migration. The central hypothesis of this proposal is that cell component transferred from donor to recipient MSCs ameliorates autoimmune disease phenotypes through FAS-assembled exocytotic machinery. During the K99 phase, I will explore the efficacy of MSCT in SSc mice and uncover the interplay between immune cells and tissue-specific stem cells to elucidate the pathophysiological mechanism of SSc (Aim 1; K99). Since miRNA-151-5p has been identified to transfer from donor to recipient MSCs, I will determine the function of miRNA-151-5p in MSC differentiation and use miRNA-151-5p gene therapy in SSc mice (Aim 2; K99). During the R00 phase, I will determine the role of the exosome in MSCT by direct infusion of exosomes for disease treatment and examine the role of the exocytosis molecule Rab27a in regulation of exosome release. I will identify a novel micro-vesicle trafficking complex from MSCs and determine its function in controlling cell component reuse (Aim 3; R00). Finally, I will examine the therapeutic effect of Del-1, which can be released via exosomes from donor MSCs to restrain cytokine production and disease phenotypes (Aim 4; R00). Upon successful completion of the Specific Aims, this translational study will extend our knowledge of SSc pathogenesis and describe detailed mechanisms of MSC-based therapy. Under my primary mentorship of Drs. Hajishengallis and Shi, the training plan will provide the opportunity to extend and develop the necessary knowledge and skills within a collaborative environment. An identified outstanding mentorship team and collaborators will broaden my training in different research fields. Under their supervision and guidance, this NIH K99/R00 Award will largely improve my ability to begin my scientific career as an independent investigator.